Early Triassic Conodont Stratigraphy And Evolution In South Tibet

The Permian-Triassic mass extinction(PTME)was the most severe biotic crisis in the Phanerozoic,wiping out over 90%of marine species,causing a shift in marine ecosystem from the Paleozoic type to the Modern type.Extreme environment in the Early Triassic led to a complex process of multi-episodic recovery and the complete recovery of marine ecosystem was long delayed untill the Middle Triassic.During the PTME,only 30%of conodont species became extinction.Conodont are subjected to multiple extinction-recovery processes during the Early Triassic.Systematic research of conodonts enables us to better determine their phylogeny and establish a high-resolution biostratigraphy.Besides,it also provides a significant foundation for deciphering the mechanism and pattern of mass extinction and the aftermath recovery.Studies of Early Triassic conodonts in the Neotethys ocean are relatively weak,and the lack of high-resolution conodont biostratigraphies limit our understanding of extinction pattern and subsequent recovery,as well as the paleoenvironmental changes of mid-high latitude regions.Here three sections of south Tibet are systematical studied including Selong section,Xialong sectiona and Gyanyima section.The Selong section and Xialong section are loctaed at the marginal sea of Gondwana and Gyanyima section is situated at the seamout.Systematical conodont researches focusing on the Early Triassic in south Tibet provides an essential tempo framework by establishing a comprehensive conodont biostratigraphical sequences.Meanwhile,the evolution of Early Triassic conodont in south Tibet is discussed.Main conclusions and highlights are as follows:1.A cladistic analysis of Early Triassic conodonts are present to reexamine the current systematic classification.The trees comprise 38 Early Triassic conodont multi-element apparatus and via the method of parsimony analysis.Based on the phylogenetic time tree,a series of evolutionary lineages are supported:Parafurnishius xuanhanensis–Furnishius triserratus–Platyvillosus regularis;Scythogondolella mosheri–Scythogondolella milleri;Discretella–Conservatella–Paullella meeki;Triassospathodus homeri–Chiosella;and Triassospathodus symmetricus–Triassospathodus chioensis.Reassignment of Ellisonia agordina to be Hadrodontina agordina is supported while Neospathodus planus should be resigned to the genus Triassospathodus.Meanwhile,Parafurnishius and Furnishius may belong to Gondolellidae rather than Ellisonidae or Ellisonidae may be a polyphyletic group rather than a monophyletic group.The cladogram suggests that Hindeodus,Hadrodontina and Pachycladina are monophyletic,whilst Novispathodus and Triassospathodus are paraphyletic and polyphyletic,respectively.In addition,a P₁element with a round basal cavity is an important trait to distinguish Novispathodus from Triassospathodus.2.Abundant conodont fossils,including 58 species in 16 genera,have been found in three sections of south Tibet.Combined with the previous work of Tulong section,a comprehensive conodont zones were established,including two Griesbachian hindeodid zones,Hindeodus parvus zone and Isarcicella staeschei zone,three Griesbachian gondolellid zones,Clarkina taylorae zone,Clarkina krystyni zone and Neoclarkina discreta zone,three Dienerian zones,Sweetospathodus kummeli zone,Neospathodus dieneri zone and Neosapthodus cristagalli zone,two Smithian zones,Novispathodus waageni zone and Scythogondolella milleri zone,two Spathian planate type zones,Columbitella elongatus zone and Columbitella joanae zone,two blade-like type zones,Novispathodus pingdingshanensis zone and Triassospathodus homeri zone,one Anisian zone,Chiosella timorensis zone.3.The Scythogondolella bloomed during the Dienerian in Selong section and a continuous evolutionary linage emerged.Similar conodont faunas have only been reported from British Columbia of Canada.However,Scythogondolella fauna in British Columbia is from the Smithian a much higher stratigraphic horizon than that of Selong section.This study suggests that the Scythogondolella fauna might first occurred in Neotethys ocean and gradually spreaded to Canada and other regions.More fossil records are still needed to confirm the specific migration trajectory.4.The Early Triassic conodonts of south Tibet suffered a multiple extinction and recovery processes including five stages,consistent with the general evolutionary pattern worldwide.Conodonts underwent a fauna turnover in late Griesbachian,while all Achignathodontidae went extinction.Gondolellidae recovered in Dienerian with increased richness of genera and species increased.A major extinction of conodonts occurred in late Smithian.During early-middle Spathian,conodonts experienced an explosive radiation and nearly all Early Triassic conodont died out in end-Spathian extinction event.5.Kangshare Formation can be divided into five members according to its lithology.Members?,?and?are carbonate deposits while members?and?are shales.Member?is dominated by dolomite and limestone and ranges from Hindeodus parvus zone to Neospathodus dieneri zone.Member?is mainly restricted into Neospathodus dieneri zone and Induan-Olenekian boundary is situated at the top part of this member.Member?is from Novispathodus waageni zone to the base of Novispathodus pingdingshanensis zone.Member?is limited in Novispathodus pingdingshanensis zone.Member?starts form Novispathodus pingdingshanensis zone to Chiosella timorensis zone.6.The dolomite deposition in south Tibet generally starts from Hindeodus parvus zone and is restricted into the Griesbachian,except for the Gongpu section which continued into the Dienerian.Red bed at Gyanyima section occurs from Smithian Novispathodus waageni zone to the late Anisian while it is limited in the Spathian at Tulong and Xialong sections.